sbomnix/meta.py: Allow None in nixpkgs path

Gracefully handle the case where nixref_to_nixpkgs_path return None.

Signed-off-by: Henri Rosten <henri.rosten@unikie.com>

src/sbomnix/meta.py

@@ -75,7 +75,9 @@ def get_nixpkgs_meta(self, nixref=None):
         if nixref:
             # Read meta from nixpkgs pinned by nixref
             LOG.debug("Reading nixpkgs path from nixref: %s", nixref)
-            nixpkgs_path = nixref_to_nixpkgs_path(nixref).as_posix()
+            nixpath = nixref_to_nixpkgs_path(nixref)
+            if nixpath:
+                nixpkgs_path = nixpath.as_posix()
         elif "NIX_PATH" in os.environ:
             # Read meta from nipxkgs referenced in NIX_PATH
             LOG.debug("Reading nixpkgs path from NIX_PATH environment")